1. Field of the Invention
The present invention relates to a filter, an antenna sharing device, and a communication device which are operable, e.g., in a microwave band.
2. Description of the Related Art
FIG. 9 shows an example of a conventional circuit configuration for a band elimination filter. The band elimination filter 1 comprises two resonance circuit stages coupled together, and includes resonators 2 and 3 electrically connected in series with trap capacitors C1 and C2, respectively. Capacitors C3 and C4 electrically connected in parallel to the resonators 2 and 3 control the resonance frequencies of the resonators 2 and 3, respectively.
A resonance circuit comprising the resonator 2, the trap capacitor C1, and the frequency control capacitor C3 is electrically connected via a coupling coil L1 to a resonance circuit comprising the resonator 3, the trap capacitor C2, and the frequency control capacitor C4. Furthermore, capacitors C5 and C6 are electrically connected in parallel to these two resonance circuits, respectively. In FIG. 9, reference numerals 5 and 6 designate input and output terminals 5 and 6, respectively.
FIG. 10 shows an example in which the conventional circuit configuration of FIG. 9 is realized on a circuit substrate 20. The resonators 2 and 3 are xcex/4 coaxial dielectric resonators. The coupling coil L1 is formed by winding as shown. The capacitors C5 and C6 are monolithic chip capacitors.
The trap capacitors C1 and C2 and the frequency control capacitors C3 and C4 are formed with one coupling plate 11 as shown in FIG. 11. The coupling plate 11 comprises a dielectric substrate 12 and capacitor electrodes 13 to 18 formed on the front and back surfaces of the dielectric substrate 12. The capacitor electrodes 13 and 16 form the trap capacitor C1. The capacitor electrodes 14 and 17 form the trap capacitor C2. The capacitor electrodes 13 and 15 form the frequency control capacitor C3. The capacitor electrodes 14 and 18 form the frequency control capacitor C4.
Individual monolithic chip capacitors can also be used, as the trap capacitors C1 and C2, and the frequency control capacitors C3 and C4. However, in this case, variations in capacitance between the capacitors C1 to C4 are large, causing the characteristics of the filter 1 to vary considerably, as compared with capacitance-variations between the capacitors formed with the coupling plate 11.
The open end-faces of the dielectric resonators 2 and 3 are connected to the capacitor electrodes 13 and 14 of the coupling plate 11 via conductors 10, respectively. That is, the resonators 2 and 3 are electrically connected to the capacitors C1 and C3 and the capacitors C2 and C4.
As the design frequency of the filter 1 becomes higher, the required capacitances of the capacitors C1 to C4 are decreased. For example, for the filter 1 to be used in the 800 MHZ band, the capacitances of the capacitors C1 to C4 are set at 1.5 to 3.0 pF. On the other hand, for the filter 1 to be used in the 1.5 GHz band which is a quasi-microwave band, the capacitances of the capacitors C1 to C4 are set at 1.0 pF or lower.
In the case in which the capacitors C1 to C4 are formed with the coupling plate 11, an unnecessary capacitance Cs is generated between the capacitor electrodes 13 and 16, and the capacitor electrodes 14 and 17. When the filter 1 is used in the 800 MHZ band, the unnecessary capacitance Cs is very small as compared with the capacitances of the capacitors C1 to C4, and exerts no influence over filter characteristics.
However, when the employed frequency of the filter 1 becomes high, the required capacitances of the capacitors C1 to C4 decrease. Therefore, the unnecessary capacitance Cs considerably affects the filter characteristics. In the case of the band-block filter 1, an unnecessary coupling is caused between the resonance circuits. Thus, there arises the problem that the attenuation characteristics of the filter deteriorate and so forth.
Accordingly, it is an object of the present invention to provide a filter, an antenna sharing device, and a communication device in which generation of an unnecessary capacitance between resonance circuits is suppressed.
To achieve the above object, according to the present invention, there is provided a filter which comprises plural resonance circuits each having a resonator, a trap capacitor, and a frequency control capacitor electrically connected to the resonator, respectively, the trap capacitor and the frequency control capacitor for each respective resonance circuit being made up of one corresponding capacitor plate. Preferably, a voltage-controllable reactance element such as a PIN diode, a field-effect transistor, a variable capacitance diode, or the like is electrically connected between each of the frequency control capacitors and ground.
The trap capacitor and the frequency control capacitor for each reactive resonance circuit are made up of one capacitor plate. Thereby, an unnecessary capacitance between the resonance circuits can be suppressed, which prevents the resonance circuits from being coupled.
The antenna sharing device and the communication device of the present invention include the filter having the above-described characteristics. Therefore, coupling can be suppressed between the resonance circuits, which provides excellent frequency characteristics.
Other features and advantages of the invention will be understood from the following description of embodiments thereof.